Influence of Ions on Aqueous Acid–Base Reactions

Cox, M. Jocelyn; Siwick, Bradley J.; Bakker, Huib J.

doi:10.1002/cphc.200800406

Cited by 23 publications

(29 citation statements)

References 44 publications

(171 reference statements)

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“…Therefore, the small NO 3 − signals detected in our experiments on pure water jets indicate that we do sample their outermost interfacial layers (24,25), and confirms that most of the mass-accommodated HNO 3 diffuses in undissociated form through such layers. The fact that the production of NO 3 − ðifÞ is dramatically enhanced by inert anions on water hints at the possibility that the barrier preventing HNO 3 dissociation at the interface might be kinetic rather than thermodynamic (26,27). In summary, the results of Fig.…”

Section: Fig 1 Displays Mass Spectral Nosupporting

confidence: 51%

Anions dramatically enhance proton transfer through aqueous interfaces

Mishra

Enami

Nielsen

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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Proton transfer (PT) through and across aqueous interfaces is a fundamental process in chemistry and biology. Notwithstanding its importance, it is not generally realized that interfacial PT is quite different from conventional PT in bulk water. Here we show that, in contrast with the behavior of strong nitric acid in aqueous solution, gas-phase HNO 3 does not dissociate upon collision with the surface of water unless a few ions (>1 per 10 6 H 2 O) are present. By applying online electrospray ionization mass spectrometry to monitor in situ the surface of aqueous jets exposed to HNO 3ðgÞ beams we found that NO 3 − production increases dramatically on >30-μM inert electrolyte solutions. We also performed quantum mechanical calculations confirming that the sizable barrier hindering HNO 3 dissociation on the surface of small water clusters is drastically lowered in the presence of anions. Anions electrostatically assist in drawing the proton away from NO 3 − lingering outside the cluster, whose incorporation is hampered by the energetic cost of opening a cavity therein. Present results provide both direct experimental evidence and mechanistic insights on the counterintuitive slowness of PT at water-hydrophobe boundaries and its remarkable sensitivity to electrostatic effects.air-water interface | acid-base | catalysis | nitric acid dissociation P roton transfers (PTs) at water interfaces, such as water boundaries with air (1, 2) or lipid membranes (3), intervene in fundamental phenomena. Arguably the most important PTs are those that take place through and across water boundaries rather than in the bulk liquid. Interfacial PTs participate in the acidification of the ocean (4), the chemistry of atmospheric gases and aerosols (1,5,6), the generation of the electrochemical gradients that drive energy transduction across biomembranes (3,7,8), and in enzymatic function (9, 10) because the activation of neutral species is most generally accomplished via acid-base catalysis (11). Interfacial PT, in contrast with conventional PT in bulk water, depends sensitively on the extent of ion hydration because the density of water in interfacial layers vanishes within 1-nm (12). The acidity of hydronium at the interface, H 3 O þ ðifÞ , is therefore expected to bridge that of H 3 O þ ðgÞ , which protonates most nonalkane species in the gas-phase (13), and H 3 O þ ðaqÞ , which neutralizes only relatively strong bases in solution. Critically controlled by ion hydration in thin yet cohesive interfacial water layers that resist ion penetration, PT "on water" clearly confronts unique constraints. Species that behave as strong acids "in water" may become weak ones on water if dissociation were hindered by kinetic and/or thermodynamic factors in the interfacial region (14, 15).Herein we address these important issues and report the results of experiments in which we monitor the dissociation of gaseous nitric acid HNO 3ðgÞ molecules in collisions with interfacial water, H 2 O ðifÞ , reaction 1 (Eq. 1):[1]The Technique Experiments were conduc...

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Section: Fig 1 Displays Mass Spectral Nosupporting

confidence: 51%

Anions dramatically enhance proton transfer through aqueous interfaces

Mishra

Enami

Nielsen

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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“…, the triple negative charge of sulfonate sidechain groups in conjunction with nearby counter ions) in sample solution that may change the proton dissociation rate from HPTS to water. This could affect both the short and long time kinetics of HPTS especially concerning the interaction between the generated ion pair 53 , 54 as inert salt ions have been reported to affect the solvent network and its fluctuations on various timescales.…”

Section: Methodsmentioning

confidence: 99%

Panoramic portrait of primary molecular events preceding excited state proton transfer in water

et al. 2016

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“…119−121 In another approach, the PT dynamics is described with a model in which the PT rate coefficient decreases by the same factor for every additional water molecule separating the acid from the base. 122,123,125 For the acetate base it was found that, at a concentration of 1 M, most PT events take place in reaction complexes in which the photoacid and the acetate base are separated by two or three water molecules.…”

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confidence: 99%

Protons and Hydroxide Ions in Aqueous Systems

et al. 2016

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Understanding the structure and dynamics of water's constituent ions, proton and hydroxide, has been a subject of numerous experimental and theoretical studies over the last century. Besides their obvious importance in acid-base chemistry, these ions play an important role in numerous applications ranging from enzyme catalysis to environmental chemistry. Despite a long history of research, many fundamental issues regarding their properties continue to be an active area of research. Here, we provide a review of the experimental and theoretical advances made in the last several decades in understanding the structure, dynamics, and transport of the proton and hydroxide ions in different aqueous environments, ranging from water clusters to the bulk liquid and its interfaces with hydrophobic surfaces. The propensity of these ions to accumulate at hydrophobic surfaces has been a subject of intense debate, and we highlight the open issues and challenges in this area. Biological applications reviewed include proton transport along the hydration layer of various membranes and through channel proteins, problems that are at the core of cellular bioenergetics.

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Influence of Ions on Aqueous Acid–Base Reactions

Cited by 23 publications

References 44 publications

Anions dramatically enhance proton transfer through aqueous interfaces

Anions dramatically enhance proton transfer through aqueous interfaces

Panoramic portrait of primary molecular events preceding excited state proton transfer in water

Protons and Hydroxide Ions in Aqueous Systems

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